Preparation And Optoelectronic Properties Of Electron Acceptors Based On 5H-fluoreno[3,2-b:6,7-b']dithiophene

The unique three-dimensional spherical structure of fullerenes and their derivatives makes them isotropic,facilitates the exciton diffusion,and enables them to form independent aggregated states to promote better phase separation.It also has a strong ability to withdraw electrons.These excellent properties make it the dominant acceptor in organic solar cells.However,due to its weak absorption in the visible region and inflexible energy level regulation,the power conversion efficiencies?PCEs?of fullerene based solar cell devices were hardly improved further.Small molecule acceptors thus have become an attractive alternative to fullerenes because of their strong light absorption,easy accessibility,low cost and their tunable energy levels to match different donors.So the development of efficient novel small molecule acceptor has become a research hotspot in recent years.We therefore were inspired to chose a fluorene derivative of 5H-fluoreno[3,2-b:6,7-b']dithiophene,which was barely utilized as central block,dicyanomethylene indanone and rhodanine as end groups to synthesize two A-?-D-?-A type small molecule acceptors.The photoelectric properties of these acceptors were studied.The main contents are as following:1.A novel small molecule acceptor TFDTBR was synthesized by using 5H-fluoreno[3,2-b:6,7-b']dithiophene as the central block and 3-ethyl rhodanine as the end groups.It can be well dissolved in a variety of common organic solvents.DICTFDT-OH was obtained by replacing the 3-ethyl rhodanine with dicyanomethylethylene indone units as the end groups,while the alkyl side chains attached on 5H-fluoreno[3,2-b:6,7-b']dithiophene and?-bridged thiophene units of it were octyl and hexyl,respectively.Then DICTFDT and DICTFDT-OE were obtained by changing side chains on the 5H-fluoreno[3,2-b:6,7-b']dithiophene and?bridge into 2-ethylhexyl,respectively.Among them,DTCTFDT was found to be able to form a good film for the preparation of organic solar cell devices.2.TFDTBR has a good thermal stability with a 5%weight loss temperature?T_d?of 388 ^oC under N₂ atmosphere.It has a strong and wide absorption in the visible region with an optical band gap of 1.69 eV.The relatively higher LUMO level?-3.44 eV?of TFDTBR helps the device achieve a high Voc.When blended with the donor polymer of PTB7-Th at a D:A mass ratio of 2:3,a Voc as high as 0.920 V was obtained in the bulk heterojunction polymer solar cell device.However due to its lower J_sc(12.17 mA cm^-2)and FF?0.35?,the maximum PCE can only reach 3.95%.3.DICTFDT has an optical band gap of 1.67 eV.The HOMO and the LUMO levels are at-5.56 eV and-4.00 eV,respectively.When blended with PTB7-Th as the donor material at a D:A mass ratio of 2:3,the device presents the best performance with a Voc of 0.891 V,J_sc of 12.52 mA cm^-2,FF of 0.46,and a higher PCE of 5.12%than that of the TFDTBR device,which is mainly ascribed to its higher electron mobility and balanced charge transport.